In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence (EL). In a basic structure of such a light-emitting element, a layer containing a light-emitting substance (an EL layer) is interposed between a pair of electrodes. By applying a voltage between the pair of electrodes of this element, light emission from the light-emitting substance can be obtained.
Since the above light-emitting element is a self-luminous type, a light-emitting device using this light-emitting element has advantages such as high visibility, no necessity of a backlight, low power consumption, and the like. Further, the light-emitting device using the light-emitting element also has advantages in that it can be formed to be thin and lightweight, and has high response speed.
In the case where the above light-emitting element is used for a display panel, there are the following two methods: a method of providing subpixels in a pixel with EL layers having functions of emitting light of different colors (hereinafter referred to as a separate coloring method) and a method of providing subpixels in a pixel with, for example, a common EL layer having a function of emitting white light and color filters each having a function of transmitting light of a different color (hereinafter referred to as a white EL+ color filter method, and it is noted that the emission color of the common EL layer is not limited to white).
One of the advantages of the white EL+ color filter method is that the EL layer can be shared by all of the subpixels. Therefore, compared with the separate coloring method, loss of a material of the EL layer is small and cost for patterning can be reduced; thus, display devices can be manufactured at low cost with high productivity. In the separate coloring method, it is necessary to provide a space between the subpixels to prevent mixture of the materials of the EL layers in the subpixels. In contrast, the white EL+ color filter method has another advantage in that the space is not necessary and therefore a high-resolution display device having higher pixel density can be achieved.
The light-emitting element can emit light of a variety of colors depending on the kind of light-emitting substance included in the EL layer. A light-emitting element which can emit white light or light of color close to white with high efficiency has been particularly required to be applied to a lighting and a display device utilizing the white EL+ color filter method. In addition, a light-emitting element having low power consumption is required.
In order to improve the extraction efficiency of light from a light-emitting element, a method has been proposed, in which a micro optical resonator (microcavity) structure utilizing a resonant effect of light between a pair of electrodes is used to increase the intensity of light having a specific wavelength (e.g., see Patent Document 1).
Furthermore, in order to reduce power consumption of a light-emitting element, a method has been proposed, in which a metal oxide having a high work function is used for one of a pair of electrodes, through which light is not extracted, to reduce voltage loss due to the electrode and thus to reduce the drive voltage of a light-emitting element (e.g., see Patent Document 2).